The present invention relates to a device for the non-destructive evaluation of service-exposed steel structures and/or components. More specifically, the invention relates to a device for the non-destructive evaluation of such structures and components as are in service and which are intended to continue in use for an extended period during which time a number of defects such as residual stress, fatigue, creep or the formation of magnetic phase in non-ferromagnetic steel can develop. Thus, in addition to its primary purpose of detecting and/or determining the stated defects in in-service steel structures or components, the device of the present invention also possesses the advantage that it can be applied for the sorting and classification of steels of different compositions according to the results of defects detected.
The determination of the presence of residual stress in critical areas of in-service components is most desirable since the undetected presence of such stresses can cause failure of such components or structures. Likewise, the determination of micro-structural degradation within in-service steel structures and components is invaluable as an early warning system to guard against sudden failure of those components and structures. Most of all, the presence of magnetic phase above a critical percentage in certain non-magnetic stainless steel structures or components is to be avoided at all costs for the reason that such magnetic phase may cause brittle fracture of the structure or component in question. It is, therefore, accepted that the determination of the presence or absence of such magnetic phase is vital.
In the past, the determination of the presence of magnetic phase, residual stress or micro-structural degradation in steel structures or components has most often been effected by employing an x-ray diffractometer and/or by metallographic analysis of the steel in question. Another technique for the magnetic non destructive evaluation of ferromagnetic materials is what is known as the Magnetic Barkhausen Emissions (MBE) technique. Unfortunately, all these methods for the determination of the presence of magnetic phase suffer from serious drawbacks.
For instance, to attempt to effect on site experimentation with an X-ray diffractometer is at the best of times not easy and on some occasions quite impossible. Portable X-ray diffractometers currently available for the measurement of residual stress in components are very expensive to purchase. What is more, although described as xe2x80x9cportablexe2x80x9d, these diffractometers are not easily transportable from one site to another and their use can be rendered totally impossible by their inaccessibility to the location of the components to be investigated.
Metallographic analysis for the quantitative estimation of the presence of a low volume percent of ferromagnetic martensite phase is not only difficult but also inaccurate and the presence of such a low percentage cannot be assessed quickly at site.
The third possible non-destructive technique for determination of ferromagnetic phase, namely the MBE technique, suffers from the shortcoming that it is not sensitive when the presence of martensite is less than 20%.
Techniques for the non-destructive evaluation of ferromagnetic materials have been fairly widely described in the prior art. Thus, magnetic techniques for determination of the degradation of components based on the hysteresis properties of the material of such components have been disclosed in U.S. Pat. No. 5,059,903 entitled xe2x80x9cMethod and apparatus utilizing a magnetic field for detecting degradation of metal materialsxe2x80x9d and in U.S. Pat. No.5,142,227 for xe2x80x9cMethod and apparatus for measuring strain within a ferromagnetic material by sensing change in coercive forcexe2x80x9d. However, magnetic hysteresis technique is not particularly sensitive to the presence of a very low amount of magnetic phase. For example, such technique is not suitable for the determination of less than 20% martensite in work hardened AISI 304 stainless steel. Moreover, it requires a power source to energise the electromagnet employed by the technique.
U.S. Pat. No. 5,166,613 entitled xe2x80x9cMethod and apparatus for mapping stress within ferromagnetic materials by analysing Barkhausen noise formed by introduction of magnetic fieldsxe2x80x9d and U.S. Pat. No. 5,121,058 entitled xe2x80x9cMethod and apparatus for useing magneto-acoustic remanence to determine embrittlementxe2x80x9d reveal that the embrittlement of steel can be studied by means of the MBE technique or magneto-acoustic emissions technique as referred to therein. However, each of these techniques requires thorough signal analysis and hence the equipment involved is both costly and complicated. Moreover, the sensitivity of the devices disclosed in these two U.S. Patents is low.
On the other hand, extremely sensitive equipment, e. g. the superconducting quantum interference device (SQUID), for determination of small changes in magnetic properties is known and has been described in U.S. Pat. No. 5,729,135 entitled xe2x80x9cNon-Destructive Testing Equipment Employing SQUID Type Magnetic Sensor In Magnetic Shield Containerxe2x80x9d and in U.S. Pat. No. 5,854,492 entitled xe2x80x9cSuperconducting Quantum Interference Device Fluxmeter And Non-Destructive Inspection Apparatusxe2x80x9d. However, SQUID is a very expensive device which requires liquid helium or liquid nitrogen as coolant for its operation and is not truly portable for field work.
An overall object of the present invention is to provide a device for the non-destructive evaluation of the presence of magnetic phase in steel structures or components which device overcomes the deficiencies and shortcomings of hitherto known devices for this purpose.
Within such overall framework, it is a basic object of the invention to provide a device for the non-destructive evaluation of the presence of magnetic phase in steel structures or components which in their virgin state are non-magnetic but which acquire a magnetic presence in the course of extended periods in service.
A more specific object of the invention is the provision of a low cost, portable easy to use magnetic sensing device for the non-destructive evaluation of steel structures or components which have undergone an extended period of service in order to determine the extent of micro-structural degradation which has taken place or the residual stress which has been generated in said structures or components during such period of service.
A still further object of the invention is the provision of a device which while capable of the non-destructive evaluation of steel structures or components for the detection of magnetic phase also affords the advantage of use as an apparatus for the sorting and classification of steels of different composition according to the results of defects detected by said device.
Yet another object of the present invention is to provide a device which for reason of its ability to detect even very weak magnetic fields can be employed as part of a security system for detecting the movement of ferromagnetic objects taking place at varying distances.
Towards achieving the objects stated, the present invention has particularly investigated and researched the aspect of the sensor mechanism employed by prior art devices for the non-destructive evaluation of the presence of magnetic phase in ferromagnetic materials. This has led to the conclusion that an improved construction comprising a sensing core and a toroidal core with respective sets of coils optimally located with respect to such cores and the use of a predetermined material for the sensing core could overcome substantially the drawback described and provide the solution to the problems which the invention seeks to resolve.
Accordingly, the present invention provides a portable, easily operable magnetic sensing device for the non-destructive evaluation of steel structures of components which have undergone an extended period of service in order to determine the extent of micro-structural degradation or residual stress within said structures or components which comprises the combination of a probe head adapted to be located proximate the structure or component to be evaluated and signal processing means, characterised in that said probe head comprises a sensing core of nanocrystalline Fe-based material having a first pair of coils wound thereabout such that in normal condition when current flows through said first pair of coils, each half of the sensing core is symmetrically magnetised and said processing means comprises a toroidal ferrite core having a second pair of coils wound thereon, one of the terminals of each of said first pair of coils of the sensing core being earthed while the other terminals thereof are each connected respectively to one terminal of each of said second pair of coils on said toroidal core, the other terminals of the second pair of coils being joined together and connected to an excitation source capable of generation of a frequency of 5 kHz, the combination of said first and second pairs of coils constituting a primary coil, the sensed signal having different harmonics which emanates from said primary coil when the probe head is placed proximate a structure or component being evaluated in order to assess the micro-structural degradation or residual stress thereof being transmitted to the input of a secondary coil mounted on said toroidal core, the output of said secondary coil being connected to a filter means active to suppress only a first harmonic signal and to permit other harmonic signals to pass onward for measurement thereof.